1. Field of the Invention
The invention relates to AC motor drives, in general, and more particularly to a voltage-source inverter-controlled AC motor drive.
2. Description of the Prior Art
When matching the ratings of the inverter with the rating of the motor, there is a need to meet the needs of a higher pulse number, a large horse power output, or a higher voltage system, as well as for providing harmonic cancellation. In so doing, manufacturing design and cost requirements strongly advocate the use of units of standard characteristics which are themselves grouping and accommodating standard parts and components. In this regard, it has been proposed to assemble in series a plurality of single phase inverters in order to put together a high voltage inverter system. See pending patent application Ser. No. 871,047, filed on June 6, 1986, W.E. 52,598 for "High Voltage Modular Inverter".
One of the problems in assembling inverters in series is to restrict the number of transformers in order to limit additional cost and losses.
The present invention calls for a 12-pulse scheme, namely a combination of two six-pulse systems having their outputs at 30.degree. to one another. The state of the art is illustrated by U.S. Pat. No. 3,750,004 of Loren H. Walker; U.S. Pat. No. 4,063,143 of W. Forstbauer; and by the article "Double Three-Phase Wound Synchronous Machine With Twelve-Pulse Rectifier Load" by Matsumoto and Inami in Electrical Engineering in Japan, Vol. 100, No. 3, 1980, pp. 249-256. The Forstbauer patent shows two transformers arranged in series in a costly and cumbersome approach when combining the two inverters upon a single output. The Matsumoto article relates to a synchronous machine as the load, with two stators at 30.degree. forming the outputs, a limitation for broader applications.
The Walker patent shows a twelve-pulse inverter drive system in which each inverter output has a primary winding, the primaries being coupled to two secondary winding, thereby to add the output voltages. By so doing, the Walker patent in fact makes use of two transformers added to the respective inverters.